1. Field of the Invention
This invention relates to a high temperature resistant lighting device. More particularly, the present invention relates to a high temperature resistant lighting device adapted for lighting a wall surface of a high temperature furnace working at more than about 500.degree. C. or more than about 1000.degree. C.
2. Description of the Prior Art
Due to safety requirements in high temperature furnaces or the like for use in iron mills, for example, it is necessary to regularly monitor the state of an inner wall of the high temperature furnace at work and check if an abnormal condition such as a damaged surface happens to be present in the inner wall. If it would be necessary that the high temperature furnace is cooled down to room temperature prior to monitoring the inner wall of the furnace, the operation rate of the furnace would be very poor. It is therefore desirable to carry out a checkup of the inner wall of the furnace under the condition where the interior of the furnace in operation is maintained at a high temperature. To observe the interior of the high temperature furnace, there are required a light source having a color temperature higher than a color temperature equivalent to the internal temperature of the furnace to lighten the wall surface of the furnace at an intensity of illumination higher than the brightness of the wall surface of the furnace and a TV camera or other means for observing the state of the wall surface. One of the lighting devices which meets the above requirement is a lighting device using a light guide or that using a floodlamp.
The light guide generally comprises a bundle of optical fibers which are disposed to extend into a metallic protective casing for introduction of light beams from a light source. The light source which emits light into one end of the light guide is typically a tungsten halogen lamp. Since a typical example of the light guide is a bundle of three hundred optical fibers having an outer diameter of about 100 .mu.m, it is practically impossible to lighten clearly the wall surface of the furnace because of a deteriorated efficiency of incident light running from the light source to the runs of the fibers and an insufficient amount of outgoing light results. To improve the input efficiency of light from the light source to the fibers, increasing the cross sectional area of the light guide has not been effective. In the event that a light guide with a sufficient efficiency of incident light is made practically possible to manufacture, a compact lighting device is not easily available because the light guide itself is bulky and voluminous.
The lighting device relying upon a floodlamp, on the other hand, is adapted for use of a xenon lamp, for example, to floodlight directly the wall surface of the furnace working at high temperatures with light beams from the lamp, without the light guide having trouble enriching the efficiency of incident light from the light source. A TV camera or other means of inspecting to pick up reflected light beams from the wall surface is used with the xenon flood lamp for inspecting purposes. Though the floodlamp type lighting device has no problem of efficiency of incident light from the light source as experienced with the light guide, it faces another problem difficult from a heat-proof point of view because it is difficult to place the light source into the high temperature atmosphere of the furnace. Hence, a separate path is required in the furnace for light traveling from the light source to the wall surface of the furnace. To lighten a plurality of desired points in the furnace, a plurality of light paths as mentioned above are needed in the furnace. This requirement makes observation of the desired points on the inner wall of the furnace practically impossible.